Yep. leaf pretty much summed it up. Any cowfish is potentially toxic, and they're all get too large to be suitable for nanos on top of it.
Two percs will likely pair up once one is significantly larger than the other. Three isn't a viable number, once two pair up, they'll attack the third. In a large tank, they've got lots of room to stay out of each other's way, in a nano the third one will almost certainly be killed.
On top of that a 5g nano simply isn't enough room for more than a couple very small fish.
You might get away with three, if you chose differently. Say one perc, one cleaner goby, and one firefish.
You're also more likely to manage it if you get rid of the two small filters and get the largest HOB filter that will fit on the back of it. Overfiltration, and regular weekly water changes are the best way to keep nanos stable.
B